Abstract

Animals employ mechano-sensory systems to detect and explore their environment. Mechano-sensation encompasses stimuli such as constant pressure, surface movement or vibrations at various intensities that need to be segregated in the central nervous system. Besides different receptor structures, sensory filtering via intrinsic response properties could provide a convenient way to solve this problem. In leech, three major mechano-sensory cell types can be distinguished, according to their stimulus sensitivity, as nociceptive, pressure and touch cells. Using intracellular recordings, we show that the different mechano-sensory neuron classes in Hirudo medicinalis differentially respond supra-threshold to distinct frequencies of sinusoidal current injections between 0.2 and 20 Hz. Nociceptive cells responded with a low-pass filter characteristic, pressure cells as high-pass filters and touch cells as an intermediate band-pass filter. Each class of mechano-sensory neurons is thus intrinsically tuned to a specific frequency range of voltage oscillation that could help segregate mechano-sensory information centrally.

Intrinsic frequency response profile of N cells to sinusoidal current injections. (A) Fluorescent labeling of a recorded N cell was used to identify the cell location post-hoc by comparison to a schematic drawing of the known location of leech sensory neurons. (B) Square pulse current injection just below (left) and above (right) the action potential threshold. Action potential waveform shows the N cell characteristics long after hyperpolarization. (C) Voltage responses to different sinusoidal stimulation frequencies: left, 0.5 Hz; middle, 2 Hz; right, 10 Hz. (D) Number of supra-threshold responses summed over the 10 sinusoidal cycles as a function of stimulation frequency. Each symbol represents the response of a single cell (n=9). (E) Action potential firing rate from which the stimulation rate was subtracted is plotted as a logarithmic function of the stimulation frequency. The dotted zero line indicates the same firing rate as the number of stimulation cycles, hence faithful firing. The solid line represents the zero action potential line, where no supra-threshold response was elicited at any time during stimulation. Symbols are as in D. Inset, magnified low stimulation frequencies in a linear graph.

Intrinsic frequency response profile of P cells to sinusoidal current injections. (A) Fluorescent labeling of a recorded P cell was used to identify the cell location post-hoc by comparison to a schematic drawing of the known location of leech sensory neurons. (B) Square pulse current injection just below (left) and above (right) the action potential threshold. The rapid onset action potential is characteristic of P cells. (C) Voltage responses to different sinusoidal stimulation frequencies: left, 1 Hz; middle, 5 Hz; right, 10 Hz. (D) Number of supra-threshold responses summed over the 10 sinusoidal cycles as a function of stimulation frequency. Each symbol represents the response of a single cell (n=8). (E) Action potential firing rate from which the stimulation rate was subtracted is plotted as a logarithmic function of the stimulation frequency. The dotted zero line indicates the same firing rate as the number of stimulation cycles, hence faithful firing. The solid line represents the zero action potential line, where no supra-threshold response was elicited at any time during stimulation. Symbols are as in D.

Intrinsic frequency response profile of T cells to sinusoidal current injections. (A) Fluorescent labeling of a recorded T cell was used to identify the cell location post-hoc by comparison to a schematic drawing of the known location of leech sensory neurons. (B) Square pulse current injection just below (left) and above (right) the action potential threshold. The initial burst of action potential is characteristic of T cells. (C) Voltage responses to different sinusoidal stimulation frequencies: left, 1 Hz; middle, 0.5 Hz; right, 10 Hz. (D) Number of supra-threshold responses summed over the 10 sinusoidal cycles as a function of stimulation frequency. Each symbol represents the response of a single cell (n=9). (E) Normalized number of action potentials as a function of stimulation frequency. The dotted horizontal line indicates half maximal firing. Symbols are as in D. (F) Action potential firing rate from which the stimulation rate was subtracted is plotted as a logarithmic function of the stimulation frequency. The dotted zero line indicates the same firing rate as the number of stimulation cycles, hence faithful firing. The solid line represents the zero action potential line, where no supra-threshold response was elicited at any time during stimulation. Symbols are as in D. (G) Normalized action potential firing in response to different sinusoidal stimulation frequencies of injected current for N (red), P (black) and T (blue) cells. Only stimulation frequencies where at least four cells were recorded are presented (mean±s.d.).